1. Field of the Invention
The present invention relates to a liquid-sealed type vibration isolator for use in a vehicle, a general industrial machine or the like and adapted to absorb and attenuate vibrations from a vibration-generating portion thereof.
2. Description of the Related Art
In an automobile engine, a vibration isolator serving as an engine mount is interposed between the engine and the chassis, so as to prevent the vibrations of the engine from being imparted to the chassis.
As a vibration isolator of this type, a liquid-sealed type vibration isolator has been proposed in which a portion of a partition wall of a pressure-receiving liquid chamber (a main liquid chamber) is formed by a diaphragm, and the diaphragm is vibrated when a limiting passageway allowing the main liquid chamber and an auxiliary liquid chamber to communicate with each other is clogged so as to suppress a change in the pressure within the liquid chambers, thereby making it possible to control an increase in the dynamic spring constant.
The diaphragm of this liquid-sealed type vibration isolator is formed of an iron plate or the like, and its periphery is supported by membrane-type rubber onto a member constituting the main liquid chamber. An electromagnet for attracting the diaphragm is disposed on the side of the diaphragm which is the side away from the main liquid chamber.
In this liquid-sealed type vibration isolator, during the occurrence of vibrations with low frequencies and relatively large amplitude, such as idling vibrations, a liquid flows between the main liquid chamber and the auxiliary liquid chamber via the limiting passageway, so that a large attenuating force is produced, thereby absorbing the vibrations with low frequencies and relatively large amplitude such as idling vibrations.
On the other hand, when the frequencies of the vibrations become high and the limiting passageway is consequently clogged, the magnetic force of the electromagnet is increased or decreased to vibrate the diaphragm in the direction in which the change in the pressure within the main liquid chamber is suppressed, thereby making it possible to control an increase in the dynamic spring constant.
In the combination of the electromagnet and the diaphragm formed of an iron plate or the like, the iron plate is moved by the attracting force of the electromagnet when the iron plate moves toward the electromagnet. On the other hand, the iron plate is moved by the restoring force of the membrane-type rubber supporting the diaphragm when the iron plate moves in a direction away from the electromagnet.
When the diaphragm is moved toward the electromagnet, the attracting force can be controlled by controlling the magnetic force of the electromagnet. However, since the movement of the diaphragm in the direction away from the electromagnet is based on the restoring force of the membrane-type rubber, the force is weak and is not efficient. For this reason, when the frequencies of the vibrations become high, the movement of the diaphragm in the direction away from the electromagnet cannot follow the inputted vibrations, so that the vibration-absorbing efficiency declines.
In addition, the change of the rubber supporting the diaphragm with the lapse of time is unavoidable, and there is a possibility that the restoring force necessary for moving the diaphragm gradually declines, resulting in the deterioration of the characteristics.